The invention concerns a sensor system as well as a manufacturing process and a self-testing process for the same. The sensor system serves for the detection of thermal radiation. Various arrangements are known for the detection of thermal radiation, especially infrared radiation (IR-radiation). FIG.1 shows the principal construction schematically. Two (or more) sensor elements 10 are attached to a substrate 15. An image of the thermal or IR-radiation given off by a source of thermal radiation 19 is formed on the detection surface of the sensor elements 10, for example, by a lens 14. The radiation is imaged by this arrangement on one of several sensor elements 10, so that a resolution according to different spatial-angle zones is possible in proportion to the number of sensor elements 10.
Such systems have the disadvantage that thermal inductive disturbances or cross talk can result due to heat conducted between the individual sensor elements 10. This means that a sensor element 10, which is not illuminated by (optically imaged) infrared radiation, will deliver a signal nevertheless, because it receives heat from neighboring sensor elements 10 which are irradiated by thermal radiation. A sensor element can be thereby warmed, for example, by several tenths of a degree. This heat can spread to an unirradiated neighboring element and there lead likewise to an output signal. The contrast of the sensor system is thus diminished. Moreover, it is thus far not possible to check the functional capability of the sensor system (including all individual sensor elements 10) easily during operation.
The application of thermopiles to a carrier membrane which is a poor conductor of heat and is stretched over cavities etched in the carrier substrate is known from U.S. Pat. No. 3,801,949. The cavities serve for the thermal insulation of the sensor element 10 with regard to the substrate 15 and thus to increase the sensitivity of the sensor system. But cross talk is detected even with this design, so that the heat insulation of the individual sensor elements with respect to one another is not sufficient. Moreover, only a comparatively small part of the substrate surface is actually covered with sensor elements 10, because the etched cavities, formed from the back side of the substrate, do not have vertical walls, which necessitates wide spacing between the individual sensor elements attached to the substrate. The detection sensitivity thus becomes too low, and relatively small point sources can become images on places between the sensor elements, as a result of image formation by the lens, so that they are not detected, and the sensor system will thus work unreliably.
FIG. 2 shows arrangements, in section as well as in the top view which is not to scale, as they are known in the state of the art. In FIG. 2A, the sensor element 10 is positioned over the etched cavity 24, which has sloping walls. The sloping walls result due to the influence of the crystal orientation in the substrate 15 upon the known manufacturing process. The sensor elements 10 are thereby widely spaced from one another, so that the density with which the surface is filled is low and the detection reliability not satisfactory. FIG. 2B shows an embodiment where the etched cavities 25 have a rhomboid outline, with walls which are vertical in the direction of thickness. The rhomboid outline arises in the case of this embodiment likewise as a result of crystal orientation. FIG. 2C shows an embodiment, in which cavities 26 are formed by an etching process from the front side of the substrate. These cavities 26 also exhibit sloping walls 21, so that the mutual spacing of the sensor elements 10 themselves is relatively wide. FIG. 2D, finally, shows an embodiment, in which a gap 23 is formed between the sensor element 10 and substrate 15, by first applying a sacrificial layer and then removing it after formation of the sensor element 10. Due to the small distance, the thermal insulation of the sensor element relative to the substrate 15 is poor, so that the signal amplitude and thus the sensitivity of the sensor system is low. Procedures based upon sacrificial layers are described, for example, in DE 19,539,696 A1 or in EP 0,534,768 or in PCT/EP89/01082. Processes making use of anisotropic etching behavior are described in EP 0,640,815 A1 or PCT/AU91/00162. In the case of the anisotropic etching process, the achievable packing density is restricted by the crystalline structure. The sacrificial-layer process results in high packing densities and low cross-talk levels. But because the tub or cavity depth is technologically limited to only a few .mu.m, the thermal insulation of the sensor elements 10 and thus the signal amplitude are on the whole unsatisfactory.